1. Field of the Invention
The present invention relates to an LLC converter including a series arrangement of a first parallel arrangement of a first switch and a first diode, and a second parallel arrangement of a second switch and a second diode. The series arrangement is coupled between a first input terminal and a second input terminal for receiving a DC input voltage, a voltage on the first terminal being positive with respect to a voltage on the second terminal, the cathode of the first diode and the cathode of the second diode being directed towards the first input terminal. The LLC converter further includes a series arrangement of a capacitor, a first inductor and a second inductor coupled in parallel to either the first diode or the second diode, a rectifier coupled to either the first inductor or the second inductor for supplying an output voltage, and a control circuit. The control circuit includes a switching controller for controlling the frequency of an on and off switching of the first switch and the second switch, and a difference detector for detecting a difference between the output voltage and a reference value, and for controlling the switching controller on the basis of this difference. The first diode and the second diode may be intrinsic to the first switch and the second switch, respectively.
The invention further relates to a method for controlling an LLC converter having switches for generating a current in a resonant circuit formed by a series arrangement of a capacitor, a first inductor, and a second inductor, the method including the steps of sensing an output voltage of the converter for obtaining a first control signal, and sensing the current of the converter.
2. Description of the Related Art
In the field of Switched Mode Power Supplies (SMPSs), the use of LLC converters is spreading. An LLC converter is characterized by the series arrangement of a capacitor (C), a first inductor (L) and a second inductor (L) for providing an output voltage. Examples of devices in which an LLC converter is applied are lighting devices, medical systems, displays systems, etc.
The topology of the LLC converter has a number of advantages, such as, a low Electromagnetic Interference (EMI), and a high efficiency. However, in practice, a problem arises in that controlling the output voltage of the converter by adjusting the switching frequency of the switches may lead to large instabilities, as will be explained below.
In a practical embodiment of an LLC converter, the switches are electronic switches, such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). Only one of the first switch and the second switch may conduct at any given time to avoid short-circuiting the power supply supplying the DC input voltage. The theoretical on-time of each of the first and second switches is 50%, assuming that both switches are driven symmetrically. In practice, the on-time of the switches will be less than 50% due to transition phenomena occurring during the off-on transition and the on-off transition of the switches.
The LLC converter has a resonance frequency which is determined by the values of the inductors, the capacitor, and external conditions which are governed by the input and output voltages and the load condition.
When the converter is driven below the resonance frequency, the current through the switched-off first switch was negative just before the moment of switching-off. As a result, the first diode parallel to the first switch starts conducting current. Now, switching on the second switch during the time the first diode is conducting results in high switching losses. These losses are caused by the hard recovery of the first diode, which is called hard switching. Preferably, the first and second diodes are fast (or even ultra fast) types in order to limit the switching losses in the hard switching mode. If power MOSFETs, having intrinsic (parasitic) diodes obviating the use of the first and the second diodes, are used as switches, operating the converter in the hard switching mode will most probably destroy the MOSFETs due to the very poor switching behavior of the intrinsic diodes.
Consequently, in practice, the LLC converter is driven above the resonance frequency in order to avoid hard switching. In this soft switching mode, the current through the switched-off first switch is positive just before the moment of switching-off. As a result, the voltage on the connecting node between the first and the second switches commutates, and the second diode parallel to the second switch starts conducting current. The second switch may be switched on at the moment the second diode is conducting, so virtually no switching losses occur. At such operating conditions, the use of MOSFETs with their intrinsic diodes is most suitable.
As to the control behavior of the LLC converter, the following is observed.
In most cases, a load should be supplied with a constant DC voltage. The LLC converter, however, generates an AC voltage which is to be rectified and filtered. A simple rectifier arrangement is a full-bridge rectifier having four diodes, connected in parallel with a filter capacitor.
A known control circuit for an LLC converter includes a series connection of a sensing circuit which generates a voltage by measuring the output voltage and/or output current of the converter, a voltage-to-frequency converter, and a driving stage for driving the switches of the converter. Additionally, an over-current protection circuit may be added. The control circuit may be designed such that the operating frequency is limited to ensure that the converter can only be operated in a predetermined frequency window avoiding, e.g., hard switching.
When frequency control is applied to the LLC converter, the output power, output voltage or output current can be controlled by controlling the switching frequency of the switches of the converter. As elucidated above, an LLC converter using power MOSFETS as switches normally is operated above the resonance frequency. In this operating area, the output power, output voltage or output current will decrease with increasing switching frequencies, whereas the output power, output voltage or output current will increase when the switching frequency is decreased.
At the moment the LLC converter is stabilized for a given supply and load condition at a frequency F, the effective voltage across the resonance capacitor C has reached a value V. By way of example, as the load increases, resulting in an increased output current, the frequency setting of the converter is decreased in order to fulfil the new increased load and supply conditions. This reduction of frequency causes an instability problem, since the output power of the LLC converter is correlated to the effective voltage across the capacitor C multiplied by the switching frequency of the first and second switches. By decreasing the switching frequency of the converter, the output power will temporarily decrease, since the effective voltage V across the capacitor C has not changed yet. The control logic will react by again decreasing the frequency to a value which is too low. On the other hand, after some time the effective voltage across the capacitor C will increase to a value greater than V due to the new operating condition, which is closer to the resonance frequency, so more power can be generated. However, as a result of the undershoot of the frequency control, an overshoot will occur, and the frequency must be increased again. Finally, the resonant voltage V across the capacitor C will stabilize on a new higher value if the instabilities are being damped. As a result, the increased output power can be generated, but there is a certain transition time during which the converter is unstable. These effects, which, with the necessary changes having been made, also play a role when, from a given stable supply and load condition, the load is decreased instead of increased, cause the instabilities in the LLC converter, and cannot be prevented when a simple frequency control is used. Under worst case conditions, the LLC converter remains unstable after a load transition, or is always unstable, when the LLC converter is frequency controlled.
For a fixed input and output voltage, the LLC converter can be designed such that the frequency has a substantially fixed value for varying output power and output current values. In practice, small input voltage variations occur, resulting in a rather limited frequency sweep (i.e., the difference between the minimum and maximum frequency). As a result, the loop gain (i.e., the frequency to output current ratio at a fixed input and output voltage) of the LLC converter designed in this way is extremely high. Consequently, instabilities in the frequency control may occur due to the fact that a small frequency change results in a huge output current change.
For a stable control behavior close to the resonance frequency of the converter, the control circuit would have to be adapted for every change of operating conditions. However, for most applications, this is not a feasible solution. On the other hand, frequency control can be used when the operating frequency is not close to the resonance frequency. However, in this situation, the needed frequency sweep would be large in order to cover all input and output voltage variations.
When current control is applied to the LLC converter, the output parameters of the converter can be controlled.
In Japanese Patent Application JP-A-10225122, an LLC converter is proposed which is regulated by using current control. In addition, a slope compensation method is used to improve the stability of the control circuit. Such a method is known (e.g., from R. Redl, N. O. Sokal: xe2x80x9cWhat a Designer Should Know About Current-mode Controlxe2x80x9d, The Power Electronics Design Conference, Anaheim, Calif., Oct. 15/17, 1985, pages 18-33) to improve the stability of a buck converter, which operates at a fixed frequency and is current controlled. In particular when such a buck converter is operated at a duty cycle higher than 50%, the slope compensation is essential to avoid instabilities.
In general, an advantage of current control of an LLC converter with respect to frequency control, is an improvement of the stability of the converter. A drawback of current control resides in the fact that the total current, i.e., the resonant current and the output current, is measured. This implies that the current measurement is always affected by the resonant current and by changes of the resonant current.
It is an object of the invention to provide an LLC converter, and a method for controlling an LLC converter, which avoid the instabilities in an LLC converter caused by a basic frequency control.
This object is achieved in an LLC converter, according to the invention, in which the control circuit further includes a current variation detector coupled to the difference detector for correcting the frequency in dependence on variations of a total current through the series arrangement of the capacitor, the first inductor and the second inductor. The correction provided by the current variation detector effectively prevents instabilities of the converter. As the converter frequency is shifted towards the resonance frequency, the resonance current starts increasing. The increase of the resonance current is detected by the current variation detector, which then shifts the frequency away from the resonance frequency. As a result of this corrective action, the output of the converter, e.g., the output voltage, is not affected by the start of an unstable behavior of the LLC converter. Thus, the instability is effectively damped by the control circuit of the converter according to the invention.
A simple, reliable implementation of the current variation detector includes a series arrangement of a current sensing circuit, a rectifier, a filter and a differentiator. The current sensing circuit may be adapted to measure a current directly, e.g., by a current transformer included in the converter total current circuit, or, advantageously, to measure a voltage which is associated with the current, in particular, the effective voltage across the capacitor of the converter. The rectifier may be a half-bridge of a full-bridge diode rectifier or any other suitable type of rectifier. The filter may be a low-pass RC-filter. The differentiator may be a high-pass RC-filter.
A first simple preferred embodiment of the control circuit of the converter according to the invention has the feature where the difference detector includes an error amplifier having a first input supplied with the reference value, and a second input coupled to an output of an adder having a first input supplied with the output voltage, and a second input coupled to the current variation detector.
A second embodiment of the control circuit has the feature where the difference detector includes a first error amplifier having a first input coupled to the output of a second error amplifier having a first input supplied with the reference value, and a second input supplied with the output voltage, the first error amplifier having a second input coupled to an output of an adder having a first input coupled to the current variation detector, a second input supplied with an input voltage of the converter, and a third input supplied with the output voltage. This second preferred embodiment has a relatively slow outer control loop controlling the output voltage of the converter, and generating an input signal for a relatively fast inner loop. The input and output voltages of the converter may preset the inner loop in such a way that the outer loop hardly will have to take corrective actions.
The above object of the invention is further achieved in a method, according to the invention, in which an output voltage of the LLC converter is sensed for generating a first control signal. In addition, the method includes determining the variation of the current for generating a second control signal, correcting the first control signal by the second control signal thereby forming a third control signal, and driving the switches of the converter with a frequency of on and off switching determined by the third control signal. As elucidated above by reference to control circuit components of an LLC converter, this control method provides an simple and effective stabilization of the converter.